The present invention relates to a biaxially stretched polyparaphenylene sulfide film excellent in slipping properties and an oil-impregnation property, and more in detail, the present invention relates to a biaxially stretched polyparaphenylene sulfide film having protuberances of an average height of from 0.3 to 2 .mu.m and of an average diameter of the bottom part thereof from 5 to 40 .mu.m in number of not less than 10.sup.2 /mm.sup.2 of the surface of the film, and a process for producing the biaxially stretched polyparaphenylene sulfide film, comprising the steps of melt-molding a resin of polyparaphenylene sulfide into a sheet-form or film-form, rapidly chilling the thus molded film or sheet, thereby obtaining an amorphous and non-stretched sheet or film, subjecting the thus obtained amorphous and non-stretched sheet or film to thermal treatment for from 0.5 to 20 sec at a temperature of from 150.degree. to 300.degree. C., cooling the thus treated sheet or film to room temperature and biaxially stretching the thus cooled sheet or film at a temperature of from 80.degree. to 120.degree. C.
As the industrial films (engineering plastic films), although polyethylene terephthalate films, polypropylene films, polyimide films, etc. have been exemplified, they are not sufficient as the film which can be formed by melt-processing, is excellent in various properties such as heat-resistance and is cheap in the production cost.
On the other hand, a resin of polyphenylene sulfide series (hereinafter, referred to as PPS) is excellent in heat-resistance and chemical resistance, and has been used for various uses as a material for incombustible films.
In the case where PPS films are used as a capacitor film or a base film for magnetic tape, by adding an inorganic filler, for instance, silica, carbon, glass, kaoline, calcium carbonate and calcium phosphate to PPS resin, the oil-impregnation property of the film is improved and a PPS film in which the running property of the tape is improved is produced.
However, in the case of the industrial production of the film of which the oil-impregnation property and the running property of the tape have been improved by adding the inorganic filler to PPS resin, various technical problems are caused.
For instance, although the thus added inorganic filler forms protuberances on the surface of the thus produced film, in the case where the affinity between the inorganic filler and the PPS resin is poor or the inorganic filler itself coagulates in the PPS resin, the formed protuberances vary in size. Such a situation is practically undesirable. Furthermore, the inorganic filler comes off as a white powder from the film during the running in the process of the production of the film, during the uses of the film as the magnetic tape or during the production of the capacitor resulting in an extremely inconvenient situation in the industrial processes for producing the goods.
Moreover, although the content of the inorganic filler in the base film for the magnetic tape is relatively small to PPS resin, since it has been well known that the performance of the film as the magnetic tape is lowered in the case where the unevenness of the surface of the film, which is formed by the inorganic filler, is too severe, the solution of the problem of how to make the surface of the film flat and smooth while maintaining the properties of the PPS film, such as the running property has been demanded.
On mixing the inorganic filler with PPS resin, there is a tendency that the electric specificity of the film, namely the electrical breakdown strength thereof, is reduced. Furthermore, there may be cases where a dielectric loss (tan .delta.) of the film is raised by the water-absorption of the thus mixed inorganic filler. Since the dielectric loss is raised by using a relatively large amount of the inorganic filler, the increase of the dielectric loss causes the self-generation of heat in the capacitor film resulting in the destruction of the capacitor and therefore, the solution of the problem of suppressing the increase of the dielectric loss has also been demanded.
On the other hand, the PPS film prepared without adding the inorganic filler has the defects of being poor in the running property and also poor in the oil-impregnation property.
Accordingly, a biaxially stretched PPS film, which is excellent in slipping properties, a running property and an oil-impregnation property and does not contain the inorganic filler, has been earnestly desired.
As a result of the present inventors' studies, it has been found by the present inventors that a biaxially stretched polyparaphenylene sulfide film produced by the process comprising the steps of melt-molding PPS resin into a sheet-form or film-form, rapidly chilling the sheet or film, thereby obtaining an amorphous and non-stretched sheet or film, subjecting the amorphous and non-stretched sheet or film to thermal treatment for from 0.5 to 20 sec. at a temperature of from 150.degree. to 300.degree. C., cooling the thus treated sheet or film to room temperature and biaxially stretching the thus cooled sheet or film at a temperature of from 80.degree. to 120.degree. C. has the excellent slipping properties and an oil-impregnation property, is excellent in flatness and shows a small dielectric loss, and on the basis of the finding, the present invention have been attained.